Embedded stressors in epitaxy source/drain regions

ABSTRACT

A method includes forming a semiconductor fin, forming a gate stack on the semiconductor fin, and a gate spacer on a sidewall of the gate stack. The method further includes recessing the semiconductor fin to form a recess, performing a first epitaxy process to grow a first epitaxy semiconductor layer in the recess, wherein the first epitaxy semiconductor layer, and performing a second epitaxy process to grow an embedded stressor extending into the recess. The embedded stressor has a top portion higher than a top surface of the semiconductor fin, with the top portion having a first sidewall contacting a second sidewall of the gate spacer, and with the sidewall having a bottom end level with the top surface of the semiconductor fin. The embedded spacer has a bottom portion lower than the top surface of the semiconductor fin.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No. 17/124,017, filed Dec. 16, 2020, and entitled “Embedded Stressors in Epitaxy Source/Drain Regions,” which claims the benefit Application No. 63/078,543, filed Sep. 15, 2020, and entitled “Embedded Stressor in EPI CD,” and U.S. Provisional Application No. 63/065,201, filed Aug. 13, 2020, and entitled “Embedded Stressor in EPI CD,” which applications are hereby incorporated herein by reference.

BACKGROUND

In the formation of Fin Field-Effect Transistors, source/drain regions were typically formed by forming semiconductor fins, recessing semiconductor fins to form recesses, and growing epitaxy regions starting from the recesses. The epitaxy regions grown from the recesses of neighboring semiconductor fins may merge with each other, and the resulting epitaxy regions may have planar top surfaces. The source/drain contact plugs are formed to electrically connect to the source/drain regions.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1, 2, 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B, 6A, 6B, 7A, 7B, 8A, 8B, 8C, 9A, 9B, 10A, 10B, and 10C illustrate the perspective views and cross-sectional views of intermediate stages in the formation of a Fin Field-Effect Transistor (FinFET) in accordance with some embodiments.

FIG. 11 illustrates the distribution of phosphorous and germanium in an epitaxy region in accordance with some embodiments.

FIG. 12 illustrates the distribution of phosphorous, arsenic, and germanium in an epitaxy region in accordance with some embodiments.

FIG. 13 illustrates a process flow for forming a FinFET in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,” “lower,” “overlying,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

A Fin Field-Effect Transistor (FinFET) and the method of forming the same are provided. In accordance with some embodiments of the present disclosure, a source/drain region of a FinFET is formed with an embedded stressor, so that dopant activation is improved. Furthermore, the source/drain region has a wavy top surface, so that the contact area between source/drain contact plug and the underlying source/drain region is increased, and contact resistance is reduced. Embodiments discussed herein are to provide examples to enable making or using the subject matter of this disclosure, and a person having ordinary skill in the art will readily understand modifications that can be made while remaining within contemplated scopes of different embodiments. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. Although method embodiments may be discussed as being performed in a particular order, other method embodiments may be performed in any logical order.

FIGS. 1, 2, 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B, 6A, 6B, 7A, 7B, 8A, 8B, 8C, 9A, 9B, 10A, 10B, and 10C illustrate the perspective views and cross-sectional views of intermediate stages in the formation of a FinFET in accordance with some embodiments of the present disclosure. The corresponding processes are also reflected schematically in the process flow shown in FIG. 13 .

FIG. 1 illustrates a perspective view of an initial structure. The initial structure includes wafer 10, which further includes substrate 20. Substrate 20 may be a semiconductor substrate, which may be a silicon substrate, a silicon germanium substrate, or a substrate formed of other semiconductor materials. The top surface of substrate 20 may have a (boo) surface plane. Substrate 20 may be doped with a p-type or an n-type impurity. Isolation regions 22 such as Shallow Trench Isolation (STI) regions may be formed to extend from a top surface of substrate 20 into substrate 20. The portions of substrate 20 between neighboring STI regions 22 are referred to as semiconductor strips 24. The top surfaces of semiconductor strips 24 and the top surfaces of STI regions 22 may be substantially level with each other in accordance with some embodiments.

STI regions 22 may include a liner oxide (not shown), which may be a thermal oxide formed through a thermal oxidation of a surface layer of substrate 20. The liner oxide may also be a deposited silicon oxide layer formed using, for example, Atomic Layer Deposition (ALD), High-Density Plasma Chemical Vapor Deposition (HDPCVD), or Chemical Vapor Deposition (CVD). STI regions 22 may also include a dielectric material over the liner oxide, wherein the dielectric material may be formed using Flowable Chemical Vapor Deposition (FCVD), spin-on, or the like.

Referring to FIG. 2 , STI regions 22 are recessed, so that the top portions of semiconductor strips 24 protrude higher than the top surfaces 22A of STI regions 22 to form protruding fins 24′. The respective process is illustrated as process 202 in the process flow shown in FIG. 13 . The portions of semiconductor strips 24 in STI regions 22 are still referred to as semiconductor strips. The etching may be performed using a dry etching process, wherein a mixture of HF and NH₃ may be used as the etching gases. The etching may also be performed using a mixture of NF₃ and NH₃ as the etching gases. During the etching process, plasma may be generated. Argon may also be included. In accordance with alternative embodiments of the present disclosure, the recessing of STI regions 22 is performed using a wet etching process. The etching chemical may include HF solution, for example.

In accordance with some embodiments, the fins for forming the FinFETs may be formed/patterned by any suitable method. For example, the fins may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in one embodiment, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers, or mandrels, may then be used to pattern the fins.

Referring to FIGS. 3A, 3B, and 3C, dummy gate stacks 30 are formed on the top surfaces and the sidewalls of protruding fins 24′. The respective process is illustrated as process 204 in the process flow shown in FIG. 13 . In accordance with some embodiments, a fin group for forming a FinFET may include a plurality of fins tightly grouped together. For example, the example shown in FIG. 3B illustrates a 2-fin group, and the example shown in FIG. 3C illustrates a 3-fin group. The fins in the same fin group may have spacings smaller than the spacings between neighboring fin groups.

The cross-sectional view shown in FIG. 3A is obtained from the reference cross-section A-A′ in FIG. 3C, and the vertical cross-sectional view shown in FIG. 3B is obtained from the vertical reference cross-section B-B′ in FIG. 3C. It is appreciated that although two dummy gate stacks 30 are illustrated for clarity, there may be more dummy gate stacks formed, which are parallel to each other, with the plurality of dummy gate stacks crossing the same semiconductor fin(s) 24′. Dummy gate stacks 30 may include dummy gate dielectrics 32 (FIG. 3A) and dummy gate electrodes 34 over dummy gate dielectrics 32. Dummy gate electrodes 34 may be formed using, for example, amorphous silicon or polysilicon, and other materials may also be used. Each of dummy gate stacks 30 may also include one (or a plurality of) hard mask layer 36 over dummy gate electrode 34. Hard mask layers 36 may be formed of silicon nitride, silicon carbo-nitride, or the like. Dummy gate stacks 30 also have lengthwise directions perpendicular to the lengthwise directions of protruding fins 24′.

Next, gate spacers 38 (FIGS. 3A and 3C) are formed on the sidewalls of dummy gate stacks 30. The respective process is illustrated as process 206 in the process flow shown in FIG. 13 . In accordance with some embodiments of the present disclosure, gate spacers 38 are formed of dielectric materials such as silicon carbon-oxynitride (SiCN), silicon oxy-carbon-oxynitride (SiOCN), silicon nitride, or the like, and may have a single-layer structure or a multi-layer structure including a plurality of dielectric layers.

In accordance with some embodiments of the present disclosure, gate spacers 38 are multi-layer gate spacers. For example, each of gate spacers 38 may include a SiN layer, and a SiOCN layer over the SiN layer. FIG. 3B also illustrates fin spacers 39 formed on the sidewalls of protruding fins 24′. The respective process is also illustrated as process 206 in the process flow shown in FIG. 13 . In accordance with some embodiments of the present disclosure, fin spacers 39 are formed by the same processes for forming gate spacers 38. For example, in the process for forming gate spacers 38, the blanket dielectric layer(s) that are deposited for forming gate spacers 38, when etched, may have some portions left on the sidewalls of protruding fins 24′, hence forming fin spacers 39. In accordance with some embodiments, the fin spacers 39 include outer fin spacers such as fin spacer 39A, which is on the outer side of the outmost fin in the fin group. The fin spacers 39 further include inner fin spacers such as fin spacer 39B, with the inner fin spacer being between the fins 24′ in the same fin group. Fin spacer 39C may be an inner fin spacer or an outer fin spacer, depending on whether the fin spacer has another fin on the right side of fin spacer 39C (and in the same fin group) or not. The illustrated fin spacer 39C shows an inner spacer as an example.

In FIG. 3A and subsequent figures that illustrate cross-sectional views, the level of the top surfaces 22A of STI regions 22 (FIG. 3A) may be illustrated, and semiconductor fin 24′ is higher than top surfaces 22A. Bottom surfaces 22B (FIG. 3A) of STI regions 22 are also illustrated in the cross-sectional views. STI regions 22 are locate at the level between 22A and 22B, and are not shown in FIG. 3A since they are in different planes than illustrated.

Referring to FIGS. 4A, 4B, and 4C, an etching process (also referred to as a source/drain recessing process hereinafter) is performed to recess the portions of protruding fins 24′ that are not covered by dummy gate stacks 30 and gate spacers 38. Recesses 40 are thus formed. The respective process is illustrated as process 208 in the process flow shown in FIG. 13 . FIGS. 4A and 4B illustrate the cross-sectional views obtained from reference cross-sections A-A and B-B, respectively, in FIG. 4C. The recessing may be anisotropic, and hence the portions of fins 24′ directly underlying dummy gate stacks 30 and gate spacers 38 are protected, and are not etched. The top surfaces of the recessed semiconductor fins 24′ may be higher than the top surfaces 22A of STI regions 22 in accordance with some embodiments. Recesses 40 are also located on opposite sides of dummy gate stacks 30, as shown in FIG. 3C.

In accordance with some embodiments, during the etching of protruding fins 24′, fin spacers 39 are also etched, so that the heights of the outer spacer 39A and inner spacer 39B are reduced. Fin spacers thus have heights H1 and H2 (FIG. 4B) as shown in FIG. 3B. Heights H1 and H2 may be equal to or different from each other. The etching of fin spacers 39 may be performed at the same time fins 24′ are recessed, with an etching gas(es) for etching fin spacers 39 added into the etching gas for recessing protruding fins 24′. The etching of fin spacers 39 may also be performed after fins 24′ are recessed, with an etching gas attacking fin spacers 39 being used. The adjustment of the heights of the fin spacers 39 may be performed through an anisotropic etching process.

In accordance with some embodiments of the present disclosure, the recessing of protruding fins 24′ is performed through a dry etching step. The dry etching may be performed using process gases such as C₂F₆, CF₄, SO₂, the mixture of HBr, Cl₂, and O₂, the mixture of HBr, Cl₂, O₂, and CF₂ etc., or the like. The etching may be anisotropic. In accordance with some embodiments of the present disclosure, as shown in FIG. 4A, the sidewalls of protruding fins 24′ facing recess 40 are substantially vertical, and are substantially flushed with the outer sidewalls of gate spacers 38. The sidewalls of protruding fins 24′ facing recess 40 may be on (110) surface planes of semiconductor substrate 20. Referring to FIG. 4B, the location of recesses 40, which are also the removed portions of protruding fins 24′, are shown. The dash lines also represent protruding fins 24′ that are directly underlying dummy gate stacks 30 (FIG. 4C), which are in a plane different than the illustrated plane.

FIGS. 5A, 5B, 6A, 6B, 7A, 7B, 8A, and 8B illustrate the processes for depositing epitaxy region(s) 42. In these figures and subsequent figures, figure numbers may be followed by a letter A or B, wherein letter A indicates that the corresponding cross-sectional view is obtained from a reference plane same as the reference plane A-A in FIG. 4C, and letter B indicates that the corresponding cross-sectional view is obtained from a plane same as the reference plane B-B in FIG. 4C.

Referring to FIGS. 5A and 5B, a first epitaxy layer 42A (which is also referred to as epitaxy layer L1) of an epitaxy region is deposited through an epitaxy process. The respective process is illustrated as process 210 in the process flow shown in FIG. 13 . In accordance with some embodiments, the deposition is performed through a non-conformal deposition process, so that the bottom portion of first layer 42A is thicker than the sidewall portions. This is incurred by allowing the growth on the (100) surface of semiconductor substrate 20 to be faster than on the (110) surface. For example, the ratio of the bottom thickness TB1 to the sidewall thickness TS1 may be in the range between about 1.5 and about 4. The deposition may be performed using Remote Plasma Chemical Vapor Deposition (RPCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), or the like. In accordance with some embodiments, epitaxy layer 42A is formed of or comprises SiAs. In accordance with alternative embodiments, epitaxy layer 42A is formed of or comprises SiP. In accordance with yet alternative embodiments, epitaxy layer 42A is formed of or comprises a SiAs layer and a SiP layer over the SiAs layer. The process gas for depositing epitaxy layer 42A may include a silicon-containing gas such as silane, disilane (Si₂H₆), dicholorosilane (DCS), or the like, and a dopant-containing process gas such as PH₃, AsH₃, or the like, depending on the desirable composition of epitaxy layer 42A. The chamber pressure may be in the range between about 100 Torr and about 300 Torr. Epitaxy layer 42A may have a first doping concentration (such as P) in the range between about 1×10²⁰/cm³ and about 8×10²⁰/cm³. Epitaxy layer 42A may have a first doping concentration (As) in the range between about 1×10²⁰/cm³ and about 1×10²¹/cm³. An etching gas such as HCl is added into the process gases to achieve selective deposition on semiconductor, but not on dielectric. Carrier gas(es) such as H₂ and/or N₂ may also be included in the process gas, for example, with a flow rate in the range about 50 sccm and about 500 sccm.

After the epitaxy to deposit epitaxy layer 42A, an etching (back) process is performed. The respective process is illustrated as process 212 in the process flow shown in FIG. 13 . In accordance with some embodiments of the present disclosure, the etching-back is isotropic. The etching process may be performed using an etching gas such as HCl and a carrier gas(es) such as H₂ and/or N₂. The preceding deposition process and the subsequent etching back are optimized so that epitaxy layer 42A has a desirable thickness. For example, after the etching process, bottom thickness TB1 of epitaxy layer 42A may be in the range between about 5 nm and about nm, and sidewall thickness TS1 may be in the range between about 4 nm and about 10 nm. The deposition time and the etching time may be adjusted accordingly, for example, with the deposition process lasts for about 20 seconds and about 60 seconds, and the etching process lasts for about 5 seconds and about 20 seconds.

As a result of the etching process, as shown in FIG. 5A, facets 42A-F may be formed, and the facets 42A-F extend to the top corners 24′TC of protruding fins 24′. In accordance with some embodiments, facets 42A-F are on the (111) planes of substrate 20. In accordance with other embodiments, facets 42A-F are steeper (more vertical) than the (111) planes of substrate 20.

FIG. 5B illustrates a cross-sectional view, wherein the bottom portions of epitaxy layers 42A are illustrated. The cross-sectional view shown in FIG. 5B is also obtained from the reference cross-section 5B-5B shown in FIG. 5A. In accordance with some embodiments, the top surface of the bottom portion of epitaxy layer 42A is level with or lower than the top end of outer fin spacer 39A, and lower than the top ends of inner fin spacer 39B.

Next, referring to FIGS. 6A and 6B, a second epitaxy layer 42B1 (which is also referred to as epitaxy layer L21) is deposited. The respective process is illustrated as process 214 in the process flow shown in FIG. 13 . The deposition process may be performed using RPCVD, PECVD, or the like. An n-type dopant is added into epitaxy layer 42B1. In the discussion of epitaxy layers 42B1, 42B2, and 42C (FIG. 8A), phosphorous is discussed as an example of the n-type dopants, while other n-type dopants such as arsenic, antimony, or the like, or combinations thereof, may be used. In accordance with some embodiments, epitaxy layer 42B1 includes silicon phosphorous, with the phosphorous having a second phosphorous concentration higher than the first phosphorous concentration in epitaxy layers 42A. For example, the second phosphorous concentration in epitaxy layers 42B1 may be in the range between about 8×10²⁰/cm³ and about 5×10²¹/cm³ in accordance with some embodiments. The second phosphorus concentration may be about one order or two orders higher than the first phosphorus concentration in epitaxy layers 42A. The process gas for forming epitaxy layer 42B1 may be similar to the process gases in the formation of epitaxy layer 42A, except the flow rates of the process gases may be different from the flow rates of the corresponding process gases in the formation of epitaxy layer 42A.

After the epitaxy to deposit epitaxy layer 42B1, an etching (back) process is performed. The respective process is illustrated as process 216 in the process flow shown in FIG. 13 . In accordance with some embodiments of the present disclosure, the etching process is isotropic. In accordance with some embodiments, the etching process is performed using an etching gas such as HCl, and a carrier gas(es) such as H₂ and/or N₂. In addition, a silicon-containing gas such as silane may be added in the etching gas. The addition of the silicon-containing gas results in a deposition effect, which occurs concurrently as the etching effect. The etching rate, however, is greater than the deposition rate, so that the net effect is the etching-back of epitaxy layer 42B1. The addition of the silicon-containing gas reduces the net etching rate, so that when the surface profile of epitaxy layer 42B1 is re-shaped, the thickness of epitaxy layer 42B1 is not reduced significantly. The deposition and the etching are optimized so that epitaxy layer 42B1 has a desirable thickness. As shown in FIG. 6A, the top surface of epitaxy layer 42B1 is re-shaped as having a V-shape by the etching process.

Referring to FIG. 6A again, the left top end of epitaxy layer 42B1 joins to the left top end of epitaxy layer 42A, with both top ends joining to the top end 24′TC of protruding fin 24′ on their left side. Accordingly, the topmost points of epitaxy layer 42B1 and epitaxy layer 42A are level with the top surface of protruding fin 24′. Similarly, the right top end of epitaxy layer joins to the right top end of epitaxy layer 42A, with both top ends joining to the top end 24′TC of protruding fin 24′ on their right side. Facets 42131-F may be formed as a result of etching epitaxy layer 42B1. In accordance with some embodiments, facets 42A-F are on the (111) planes of substrate 20. In accordance with alternative embodiments, facets 42B1-F are on the (m) planes of substrate 20.

Referring to FIG. 6B, the epitaxy layer 42B1 grown from neighboring recesses are merged, with air gap 44 being sealed under epitaxy layer 42B1. The top surface of the merged epitaxy layer 42B1 may have a non-planar profile (also referred to as having a wavy shape), with the middle portion between neighboring fins being lower than the portions on its opposite sides. Also, as shown in both of FIGS. 6A and 6B, the top ends of the top surfaces of epitaxy layer 42B1 are controlled to be leveled with the top surface of protruding fins 24′.

FIGS. 7A and 7B illustrate the epitaxy process for depositing a third epitaxy layer 42B2 (which is also referred to as epitaxy layer L22). The respective process is illustrated as process 218 in the process flow shown in FIG. 13 . As shown in FIG. 7B, the top surface of epitaxy layer 42B2 has the wavy shape. The deposition process may be performed using RPCVD, PECVD, or the like. In accordance with some embodiments, epitaxy layer 42B2 includes silicon phosphorous, with the phosphorous having a third phosphorous concentration higher than the second phosphorous concentration in epitaxy layers 42B1. Furthermore, epitaxy layer 42B2 has the highest phosphorous concentration in the resulting source/drain region. For example, the third phosphorous concentration in epitaxy layers 42B2 may be in the range between about 2×10²¹/cm³ and about 5×10²¹/cm³ in accordance with some embodiments. The ratio of the third phosphorus concentration to the second phosphorus concentration of epitaxy layer 42B1 may be in the range between about 3 and about 6. The process gases for forming epitaxy layer 42B2 may be similar to the process gases in the formation of epitaxy layer 42B1, except the flow rates are adjusted to achieve the desirable concentrations.

After the epitaxy to deposit epitaxy layer 42B2, an etching process is performed. The respective process is illustrated as process 220 in the process flow shown in FIG. 13 . In accordance with some embodiments of the present disclosure, the etching is isotropic. In accordance with some embodiments, the etching process is performed using an etching gas such as HCl and a carrier gas(es) such as H₂ and/or N₂. In addition, a silicon-containing gas such as silane may be added into the etching gas to deposit silicon. The etching process thus includes both of an etching effect and a deposition effect, with the net effect being etching. The addition of the silicon-containing gas reduces the etching rate, so that when the surface profile of epitaxy layer 42B2 is re-shaped, the thickness of epitaxy layer 42B2 is not reduced significantly. With the topmost ends of epitaxy layer 42B1 being in contact with the top corners 24′TC of protruding fins 24′, the top portions of epitaxy layer 42B2, which are over epitaxy layer 42B1, are higher than the top surfaces of protruding fins 24′. The sidewalls 42B2-SW of the top portions of epitaxy layer 42B2 are thus in contact with the sidewalls of gate spacers 38. The sidewalls 42B2-SW are on the (110) surface planes of the semiconductor material of epitaxy layer 42B2.

Since the material and the lattice structure of epitaxy layer 42B2 are different from the material and the structure of gate spacers 38, a stress is generated and applied by gate spacers 38 to the resulting epitaxy layers. Epitaxy layer 42B2 is an embedded stressor embedded in the resulting source/drain region. The internal stress in epitaxy layer 42B2 is a tensile stress. As shown in FIG. 6A, at least a part of the stress is contributed by gate spacers 38, and the stress is increased due to that epitaxy layer 42B2 has a high doping concentration (of phosphorus, for example). The lower part of epitaxy layer 42B2 is lower than the top surfaces of protruding fins 24′, and hence the stress is passed from the top portion of epitaxy layer 42B2, which top portion is higher than the top surfaces of protruding fins 24′, to the lower/bottom portion of epitaxy layer 42B2, which lower/bottom portion is lower than the top surfaces of protruding fins 24′. Furthermore, both of the top surface and the bottom surface of epitaxy layer 42B2 may have the V-shapes, which may improve the efficiency in transferring stress from top portion to the bottom portion of epitaxy layer 42B2. Accordingly, the stress is also applied to the channel of the resulting FinFET, and hence improves the performance of the resulting FinFET. In addition, the internal stress in the resulting FinFET also results in the increase in the activation rate of the dopant (phosphorous, for example). To maximize the stress, the height H₃ of sidewalls 42B2-SW is in a selected range. For example, height H₃ is great enough to induce a high stress. On the other hand, a too-high height H₃ results in the saturation of the stress, and may cause less stress to be passed to the lower portion of epitaxy layer 42B2. In accordance with some embodiments, height H₃ is in the range between about 3 nm and about 15 nm.

Furthermore, the depth D1 (FIG. 7A) of epitaxy layer 42B2, which is the depth of epitaxy layer 42B2 below the top surfaces of protruding fins 24′, is also in a selected range to maximize the stress received from the top part of epitaxy layer 42B2, and to maximize the effect of the stress. For example, depth D1 may be in the range between about 3 nm and about 15 nm. Furthermore, a ratio D1/H4 may be in the range between about 0.3 and about 0.5, wherein height H4 is the height of protruding fins 24′.

It is appreciated that different types of devices may have different depth D1 and height H₃ to achieve optimized stress. For example, a FinFET used in a Static Random Access memory (SRAM) cell may have depth D1 and height H3 smaller than the corresponding depth D1 and height H3 in a FinFET used in an input-output (IO) circuit. For example, an SRAM FinFET may have height H3 in the range between about 1 nm and about 10 nm, while the height H3 of the IO FinFET may be in the range between about 5 nm and about 15 nm. An SRAM FinFET may have depth D1 in the range between about 20 percent and about 30 percent of fin height H4, while the depth D1 of the IO FinFET may be in the range between about 40 percent and about 60 percent of the corresponding fin height H4.

FIGS. 8A and 8B illustrate the epitaxy process for depositing a fourth epitaxy layer 42C (which is also referred to as epitaxy layer L3 or a capping layer). The respective process is illustrated as process 222 in the process flow shown in FIG. 13 . The deposition process may be performed using RPCVD, PECVD, or the like. The top surface of epitaxy layer 42C (FIG. 8B) maintains the wavy shape. In accordance with some embodiments, epitaxy layer 42C includes silicon phosphorous, with the phosphorous having a fourth phosphorous concentration lower than the phosphorous concentrations in epitaxy layers 42B2. In addition, germanium may be incorporated, for example, with a germanium atomic percentage in the range between about 1 percent and about 5 percent. In accordance with some embodiments, the phosphorous concentration in epitaxy layers 42C may be in the range between about 1×10²⁰/cm³ and about 3×10²¹/cm³. The process gases for forming epitaxy layer 42C may be similar to the process gases in the formation of epitaxy layer 42B2, except a germanium-containing gas such as germane (GeH₄), digermane (Ge₂H₆), or the like may be added. Throughout the description, epitaxy layers 42A, 42B1, 42B2, and 42C are collectively and individually referred to as epitaxy layers 42, which are collectively referred to as source/drain regions 42 hereinafter. Source/drain regions 42 are also shown in FIG. 8C.

Referring to FIGS. 9A and 9B, Contact etch stop layer (CESL) 46 and Inter-Layer Dielectric (ILD) 48 are formed over epitaxy regions 42, and on the sides of dummy gate stacks 30 (FIG. 8A). The respective process is illustrated as process 224 in the process flow shown in FIG. 13 . A planarization process such as a Chemical Mechanical Polish (CMP) process or a mechanical grinding process is performed to remove excess portions of CESL 46 and ILD 48, until dummy gate stacks 30 (FIG. 8A) are exposed. The dummy gate stacks 30 are replaced with replacement gate stacks 56 as shown in FIG. 9A. The processes for forming the replacement gate stacks are not shown. The resulting replacement gate stacks 56, however, are shown in FIG. 9A. Replacement gate stacks 56 includes gate dielectrics, which further include interfacial layers 50 on the top surfaces and sidewalls of protruding fins 24′, and high-k dielectrics 52 on the interfacial layers. Replacement gate stacks 56 further include gate electrodes 54 over high-k dielectrics 52. After the formation of replacement gate stacks 56, replacement gate stacks 56 are recessed to form trenches between gate spacers 38. A dielectric material such as silicon nitride, silicon oxynitride, or the like, is filled into the resulting trenches to form hard masks 58.

Next, ILD 48 and CESL 46 are etched to form contact opening 60. Opening 6 o penetrates through epitaxy layer 42C, so that epitaxy layer 42B2 is exposed. The respective process is illustrated as process 226 in the process flow shown in FIG. 13 . As shown in both of FIGS. 9A and 9B, epitaxy layer 42C is etched-through, and the top surface of epitaxy layer 42B2 is exposed. The adding of germanium in epitaxy layer 42C results in the etching rate of epitaxy layer 42C to be significantly greater than the etching rate of epitaxy layer 42B2, and hence by controlling the etching process, the etching may be substantially stopped on epitaxy layer 42B2, with the over-etching of epitaxy layer 42B2 being small. As shown in FIG. 9B, the exposed top surface of epitaxy layer 42B2 is wavy, with the middle portion being recessed relative to the opposing portions on the opposite sides of the middle portion, so that the middle portion has a V-shape in the cross-sectional view.

Next, as shown in FIGS. 10A, 10B, and 10C, source/drain silicide region 64 and source/drain contact plugs 66 are formed. FIG. 10A illustrates the cross-sectional view in reference cross-section A-A in FIG. 10C. FIG. 10B illustrates the cross-sectional view in reference cross-section B-B in FIG. 10C (except FIG. 10B shows two fins, while FIG. 10C shows three fins). In accordance with some embodiments of the present disclosure, the formation of the source/drain silicide region 64 includes depositing a metal layer such as a titanium layer, a cobalt layer, or the like, extending into opening 60 (FIGS. 9A and 9B), and then performing an annealing process so that the bottom portion of the metal layer reacts with epitaxy layer 42B2 to form a silicide region. The respective process is illustrated as process 228 in the process flow shown in FIG. 13 . The remaining un-reacted metal layer may be removed. Source/drain contact plug 66 is then formed in trench 60, and is electrically connected to the respective source/drain silicide region 64. The respective process is illustrated as process 230 in the process flow shown in FIG. 13 . FinFET 68 is thus formed.

FIG. 10B illustrates some example dimensions in accordance with some embodiments. Bottom thickness TB1 of epitaxy layer 42A may be in the range between about 3 nm and about 20 nm. Height T2, which is the height of the merged epitaxy layer 42B1 relative to the bottom of protruding fins 24′, may be in the range between about 30 nm and about 70 nm. Merging height T3, which is the height of the merged portions of source/drain region 42, may be in the range between about 5 nm and about 30 nm. Height T4, which is the height of silicide regions 64, may be in the range between about 3 nm and about 20 nm. Fin height H4 may be in the range between about 40 nm and about 100 nm. The width Wi of merged epitaxy region 42 may be in the range between about 40 nm and about 100 nm.

FIG. 11 illustrates the distribution profiles of phosphorus (the left Y-axis) and germanium (the right Y-axis) in epitaxy layers 42C, 42B2, 42B4 and 42A in accordance with some embodiments. The corresponding epitaxy layer 42A is a single SiP layer in the illustrated example. The left Y-axis shows the phosphorous concentration, which is represented by line 70. The right Y-axis shows the germanium atomic percentage, which is represented by line 72. FIG. 12 illustrates the distribution profiles of phosphorus and arsenic (the left Y-axis) and germanium (the right Y-axis) in layers 42C, 42B2, 42B1, and 42A in accordance with some embodiments. The corresponding epitaxy layer 42A includes a SiAs layer and a SiP layer on the SiAs layer. The left Y-axis shows the phosphorous concentration, which is represented by line 74, and the arsenic concentration, which is represented by line 78. The right Y-axis shows the germanium atomic percentages of Ge, wherein the atomic concentration of Ge is represented by line 76.

The embodiments of the present disclosure have some advantageous features. By forming embedded stressors in source/drain regions, the dopant activation of the source/drains is improved. Furthermore, the source/drain silicide region contacts the underlying epitaxy region through a wavy interface, so that the contact area is increased compared to planar contact interfaces, and hence the contact resistance is reduced.

In accordance with some embodiments of the present disclosure, a method comprises forming isolation regions extending into a semiconductor substrate; forming a semiconductor fin protruding higher than top surfaces of the isolation regions; forming a gate stack on the semiconductor fin; forming a gate spacer on a sidewall of the gate stack; recessing the semiconductor fin to form a recess; performing a first epitaxy process to grow a first epitaxy semiconductor layer in the recess, wherein the first epitaxy semiconductor layer has a first dopant concentration; and performing a second epitaxy process to grow an embedded stressor extending into the recess, wherein the embedded stressor has a second dopant concentration higher than the first dopant concentration, and wherein the embedded stressor comprises a top portion higher than a top surface of the semiconductor fin, wherein the top portion has a first sidewall contacting a second sidewall of the gate spacer, and the sidewall has a bottom end level with the top surface of the semiconductor fin; and a bottom portion lower than the top surface of the semiconductor fin. In an embodiment, the method further comprises, after the first epitaxy process, performing an etching process on the first epitaxy semiconductor layer. In an embodiment, the etching process is performed using a process gas comprising an etching gas and silane. In an embodiment, the first epitaxy process is performed with the etching gas added. In an embodiment, at a time the second epitaxy process is started, a topmost end of the first epitaxy semiconductor layer contacts, and is level with, a top corner of the semiconductor fin, and the embedded stressor is grown starting from the topmost end upwardly to form the first sidewall. In an embodiment, the embedded stressor has a V-shaped bottom surface and a V-shaped top surface. In an embodiment, the method further comprises, after the second epitaxy process, performing a third epitaxy process to grow a second epitaxy semiconductor layer over the embedded stressor; and forming a silicide region over and contacting the embedded stressor. In an embodiment, the method further comprises, before the first epitaxy process, performing an additional epitaxy process to deposit a second epitaxy semiconductor layer in the recess, wherein both of the first epitaxy semiconductor layer and the second epitaxy semiconductor layer have a topmost end joining to a top end of the semiconductor fin.

In accordance with some embodiments of the present disclosure, a device comprises a semiconductor substrate; isolation regions extending into the semiconductor substrate; a semiconductor fin protruding higher than top surfaces of the isolation regions; a gate stack on a top surface and sidewalls of the semiconductor fin; and a source/drain region on a side of the semiconductor fin, wherein the source/drain region comprises a first semiconductor layer having a first dopant concentration; and an embedded stressor over and contacting the first semiconductor layer, wherein the embedded stressor has a second dopant concentration higher than the first dopant concentration, and wherein the embedded stressor has an upper portion higher than the top surface of the semiconductor fin, and a lower portion lower than the top surface of the semiconductor fin. In an embodiment, the device further comprises a gate spacer on a sidewall of the gate stack, wherein the upper portion of the embedded stressor contacts the gate spacer to form a vertical interface, and wherein a bottom surface of the embedded stressor is slanted, and joins to a point where a bottom of an outer surface of the gate spacer joins a top end of a sidewall of the semiconductor fin. In an embodiment, the embedded stressor has a V-shaped bottom surface. In an embodiment, the device further comprises a source/drain silicide region over and contacting the embedded stressor, wherein the source/drain silicide region has a V-shape in a cross-sectional view. In an embodiment, the embedded stressor comprises silicon phosphorous, and the device further comprises a capping layer over the embedded stressor, and wherein the capping layer comprises silicon, germanium, and phosphorous. In an embodiment, the device further comprises a second semiconductor layer under the first semiconductor layer, wherein the second semiconductor layer has a lower dopant concentration than the first semiconductor layer. In an embodiment, the first semiconductor layer has a first facet having a first top end, the second semiconductor layer has a second facet having a second top end, and wherein the first top end joins the second top end, and further joins a top corner of the semiconductor fin. In an embodiment, one of the first facet and one of the second facet is on a (111) surface plane of the source/drain region. In an embodiment, the embedded stressor comprises a V-shaped bottom surface, with a topmost point of the V-shaped bottom surface being at a same level as the top surface of the semiconductor fin.

In accordance with some embodiments of the present disclosure, a device comprises a semiconductor fin; a gate stack on the semiconductor fin; and a source/drain region on a side of the semiconductor fin, wherein the source/drain region comprises an embedded stressor, and the embedded stressor comprises a V-shaped bottom surface, wherein a top end of the V-shaped bottom surface is at a same level as a top surface of the semiconductor fin; and a V-shaped top surface, wherein a first portion of the V-shaped top surface is higher than the top surface of the semiconductor fin, and a second portion of the V-shaped top surface is lower than the top surface of the semiconductor fin. In an embodiment, the device further comprises a semiconductor layer underlying the embedded stressor, wherein the semiconductor layer comprises a facet on a (111) surface plane of the semiconductor layer. In an embodiment, the facet on the (111) surface plane extends to join a top corner of the semiconductor fin.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A device comprising: a semiconductor fin; a gate stack on the semiconductor fin; a gate spacer on a sidewall of the gate stack; and a source/drain region aside of the gate spacer and comprising: a first semiconductor layer contacting a sidewall of the semiconductor fin, wherein the first semiconductor layer has a first phosphorous concentration; a second semiconductor layer over and contacting the first semiconductor layer, wherein a first topmost tip of a first top portion of the second semiconductor layer is joined to a top corner of the semiconductor fin, and one of a top surface and a bottom surface of the first top portion has a (111) lattice plane, and the second semiconductor layer has a second phosphorous concentration higher than the first phosphorous concentration; and an embedded stressor over and contacting the second semiconductor layer, wherein the embedded stressor has a third phosphorous concentration higher than the second phosphorous concentration, and the embedded stressor comprises: an upper portion contacting the gate spacer to form a vertical interface; and a lower portion lower than the top corner of the semiconductor fin.
 2. The device of claim 1, wherein the first semiconductor layer further comprises a second top portion with a second topmost tip joined to the first topmost tip and the top corner of the semiconductor fin.
 3. The device of claim 1, wherein the top surface of the first top portion of the second semiconductor layer is on a (111) lattice plane.
 4. The device of claim 1, wherein the bottom surface of the first top portion of the second semiconductor layer is on a (111) lattice plane.
 5. The device of claim 1 further comprising a third epitaxy layer over and contacting the embedded stressor, wherein the third epitaxy layer has a fourth phosphorous concentration lower than the third phosphorous concentration.
 6. The device of claim 5, wherein the third epitaxy layer comprises germanium in addition to silicon and phosphorous.
 7. The device of claim 5 further comprising a silicide layer and a contact plug over and contacting the silicide layer, wherein the silicide layer and the contact plug collectively penetrate through the third epitaxy layer, and the silicide layer is over and contacting the embedded stressor.
 8. The device of claim 1, wherein the second semiconductor layer further comprises a curved top surface and a curved bottom surface connecting to the top surface and the bottom surface, respectively, of the first top portion of the second semiconductor layer.
 9. A device comprising: a semiconductor substrate; isolation regions extending into the semiconductor substrate; a semiconductor fin protruding higher than top surfaces of the isolation regions; a gate stack on a top surface and sidewalls of the semiconductor fin; and a source/drain region on a side of the semiconductor fin, wherein the source/drain region comprises: a first semiconductor layer having a first dopant concentration; and an embedded stressor over and contacting the first semiconductor layer, wherein the embedded stressor has a second dopant concentration higher than the first dopant concentration, and wherein the embedded stressor has an upper portion higher than the top surface of the semiconductor fin, and a lower portion lower than the top surface of the semiconductor fin.
 10. The device of claim 9 further comprising a gate spacer on a sidewall of the gate stack, wherein the upper portion of the embedded stressor contacts the gate spacer to form a vertical interface, and wherein a bottom surface of the embedded stressor is slanted, and joins to a point where a bottom of an outer surface of the gate spacer joins a top end of a sidewall of the semiconductor fin.
 11. The device of claim 9, wherein the embedded stressor has a V-shaped bottom surface.
 12. The device of claim 9 further comprising a source/drain silicide region over and contacting the embedded stressor, wherein the source/drain silicide region has a V-shape in a cross-sectional view.
 13. The device of claim 9, wherein the embedded stressor comprises silicon phosphorous, and the device further comprises a capping layer over the embedded stressor, and wherein the capping layer comprises silicon, germanium, and phosphorous.
 14. The device of claim 9 further comprising a second semiconductor layer under the first semiconductor layer, wherein the second semiconductor layer has a lower dopant concentration than the first semiconductor layer.
 15. The device of claim 14, wherein the first semiconductor layer has a first facet having a first top end, the second semiconductor layer has a second facet having a second top end, and wherein the first top end joins the second top end, and further joins a top corner of the semiconductor fin.
 16. The device of claim 15, wherein one of the first facet and one of the second facet is on a (111) surface plane of the source/drain region.
 17. The device of claim 9, wherein the embedded stressor comprises a V-shaped bottom surface, with a topmost point of the V-shaped bottom surface being at a same level as the top surface of the semiconductor fin.
 18. A device comprising: a semiconductor fin; a gate stack on the semiconductor fin; and a source/drain region on a side of the semiconductor fin, wherein the source/drain region comprises an embedded stressor, and the embedded stressor comprises: a V-shaped bottom surface, wherein a top end of the V-shaped bottom surface is at a same level as a top surface of the semiconductor fin; and a V-shaped top surface, wherein a first portion of the V-shaped top surface is higher than the top surface of the semiconductor fin, and a second portion of the V-shaped top surface is lower than the top surface of the semiconductor fin.
 19. The device of claim 18 further comprising a semiconductor layer underlying the embedded stressor, wherein the semiconductor layer comprises a facet on a (111) surface plane of the semiconductor layer.
 20. The device of claim 19, wherein the facet on the (111) surface plane extends to join a top corner of the semiconductor fin. 